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FAN6520BIM Fiches technique(PDF) 6 Page - Fairchild Semiconductor |
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FAN6520BIM Fiches technique(HTML) 6 Page - Fairchild Semiconductor |
6 / 14 page 6 www.fairchildsemi.com FAN6520B Rev. 1.0.3 Circuit Description Initialization The FAN6520B automatically initializes upon receipt of power. The Power-On Reset (POR) function continually monitors the bias voltage at the VCC pin. When the sup- ply voltage exceeds its POR threshold, the IC initiates the soft-start operation. Soft-Start The POR function initiates the soft-start sequence. Soft- start clamps the error amplifier output (COMP pin) and reference input (noninverting terminal of the error amp) to the internally generated soft-start voltage. Figure 3 shows a typical start up interval where the COMP pin has been released from a grounded (system shutdown) state. The clamp on the error amplifier (COMP pin) ini- tially controls the converter’s output voltage during soft- start. The oscillator’s triangular waveform is compared to the ramping error amplifier voltage. This generates SW pulses of increasing width that charge the output capaci- tor(s). When the internally generated soft-start voltage exceeds the feedback (FB pin) voltage, the output volt- age is in regulation. This method provides a rapid and controlled output voltage rise. The entire startup sequence typically takes about 11ms. Figure 3. Soft-Start Interval Adaptive Gate Drive The FAN6520B incorporates a MOSFET shoot-through protection method which allows a converter to both sink and source current. Care should be exercised when designing a converter with the FAN6520B when it is known that the converter may sink current. When the converter is sinking current, it is behaving as a boost converter that is regulating its input voltage. This means that the converter is boosting current into the VCC rail, which supplies the bias voltage to the FAN6520B. If this current has nowhere to go—such as to other distributed loads on the VCC rail, through a voltage limiting protection device, or other methods—the capaci- tance on the VCC bus will absorb the current. This situa- tion will allow the voltage level of the VCC rail to increase. If the voltage level of the rail is boosted to a level that exceeds the maximum voltage rating of the FAN6520B, then the IC will experience an irreversible failure and the converter will no longer be operational. Ensure that there is a path for the current to follow other than the capacitance on the rail to prevent this failure mode. Application Guidelines Layout Considerations As in any high frequency switching converter, layout is very important. Switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces. Use wide, short-printed circuit traces to minimize these interconnecting impedances. The critical com- ponents should be located as close together as possible, using ground plane construction or single point grounding. Figure 4 shows the critical power components of the converter. To minimize the voltage overshoot, the interconnecting wires indicated by heavy lines should be part of a ground or power plane in a printed circuit board. The components shown in Figure 4 should be located as close together as possible. Please note that the capa- citors CIN and COUT may each represent numerous physical capacitors. Locate the FAN6520B as close as possible to Q1 and Q2 MOSFETs. The circuit traces for the MOSFETs’ gate and source connections from the FAN6520B must be sized to handle up to 1A peak current. Figure 4. Printed Circuit Board Power and Ground Planes or Islands +VOUT Q2 Q1 CIN LOUT COUT Vin LDRV SW HDRV FAN6520B |
Numéro de pièce similaire - FAN6520BIM |
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Description similaire - FAN6520BIM |
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